The invention relates to rotating electric machinery; it relates, more particularly to electric motors and generators in which maximum mechanical power per unit volume is increased over conventional designs for comparable efficiencies at low revolution rates. Further, at elevated revolution rates, core losses in the stator as a result of alternating currents in the stator winding and alternating magnetic fields in the stator due to the rotating magnetic rotor are reduced by means of a ferrite core upon which the stator winding is fixed.
In the text hereof, the following terms will be used from time to time and their respective meanings are set forth below for convenient reference:
(1) Magnetic Flux--A characteristic of an energy field produced by a magnetomotive force. When this state is altered in magnitude, a voltage is induced in an electric conductor linked with it. The flux is thought to be a line or lines (imaginary).
(2) Magnetic Flux Density (B)--The magnitude of flux perpendicularly passing through a unit area.
(3) Saturation Magnetic Flux Density (B.sub.s)--The maximum magnetic flux density that can be induced in a material. It is the measured magnetic flux density minus that of vacuum space.
(4) Remanent Magnetic Flux Density (B.sub.r)--The Magnetic flux density of a material remaining after it has been saturated and the magnetic field intensity has been subsequently reduced to zero.
(5) Magnetic Field Intensity (H)--A characteristic of an energy field related to the magnetomotive force by a line integral. It is commonly referred to as a coercive force and is generated by current loops or a permanent magnet.
(6) Intrinsic Magnetic Field Intensity (H.sub.i)--is the magnetic field intensity required to reduce the magnetic flux density in a material to zero after it has been saturated.
(7) Magnetomotive Force (F)--The spacial distribution of the time derivative of charge whereby the magnetic field is manifested.
(8) Energy Product (BH)--A convenient unit in engineering whereby permanent magnets are compared; the product of the magnetic flux density and the magnetic field intensity at a point on the demagnetization curve of a material; units of energy per volume.
(9) Maximum Energy Product (BH.sub.max)--The product of B and H which is larger than any other point on the demagnetization curve.
(10) Energy Density (E/V)--The energy per unit volume in cgs units found by dividing the energy product by 8.
(11) Balanced Circuit--A magnetic circuit dimensioned so that the magnetomotive forces are efficaciously distributed.
(12) Operating Point--That point on the magnet demagnetization curve for which the magnetic circuit is statically balanced.
(13) Cogging--A characteristic of electrical machines employing toothed components such that torque is required to rotate the rotor a displacement of one tooth to the next-used to advantage in stepper motors.
Traditional designs employing permanent (magnetically "hard") ferrite magnets, typically barium or strontium ferrite, or rare-earth magnets, typically samarium cobalt, operate the magnet near the remanent magnetic flux density while maintaining a small gap although large enough to permit economical fabrication. The magnet flux is made to flow through a slotted iron laminated stator with the winding coils inserted into the slots. The primary difficulty with this approach appears when the width of the teeth are reduced, thus, enlarging the slots which when taken to an optimum magnetic circuit, meaning maximization of the magnet energy product, results in mechanically unsound structures. Generally, designs employing permanent ferrite or rare-earth magnets are patterned after designs employing Alnico magnets, where, in fact, the rule of thumb is to minimize the air gap length. It is a common belief that maximizing the circuit magnetic flux, which in effect is to operate near the remanent magnetic flux density, generically speaking, produces "the most powerful configuration." This approach ignores the fact that the magnets are operating far from the maximum energy product, and in fact, comparing maximum energy products between Alnico and rare-earth, suggests that a rare-earth magnet motor should be at least three times as "powerful."
The designers of rotating electric machinery have been traditionally aware of the substantial role core loss plays in the operation of electric motors and generators, such core loss resulting as heat in the stator, since the permissible temperature rise of the materials utilized to insulate the conductors in these components and the degradation of magnetic properties of magnetic materials with temperature rise sets a practical limit to the reduction in size and improvement in efficiency of their products. Furthermore, the core loss being sensitive to the frequency of the alternating current in the stator core and the frequency of the alternating magnetic fields in the stator due to the rotating magnetic rotor and enhanced as the frequency is increased sets additional limits on maximum operating frequency of the winding as well as the revolution rate of the rotor.
Not withstanding the difficulties of heat generated within the stator core, the permeability of the prior art laminated electrical iron stator core decreases as the frequency of the alternating current in the stator winding increases and the frequency of the alternating magnetic fields due to the rotating magnetic rotor increases. This results in nonlinear motor characteristics becoming exaggerated as the frequencies increase appearing as a loss of torque at high revolution rates.
Prior art is typified by design "rules of thumb" which constitute design practices based on popular belief. It is further asserted that these practices are applied to electrical rotating machines irrespective of type. In particular, they are (1) maintenance of a gap as short as possible. (2) the incompatibility of soft magnetic ferrite and permanent magnets, and (3) operation of the magnets close to the remanent point to achieve the greatest "power."
Minimizing the air gap is utilized to maintain the magnetic fields predominantly in the rotor and stator, that portion in the gap being unusable. The reluctance of the gap generally being quite large demands a significant portion of the magnet volume to sustain, as well as induction motors which also require a significant portion of the total ampere turns to sustain. Obviously, there is a spacial utility of the magnetic field based on the intent of the structure as is illustrated by the U.S. Pat. No. 2,885,645, it being an induction type machine employing a magnetically "soft" ferrite (i.e. non-permanent magnet) in both the stator and rotor. In any case, the stator structure in which the winding is embedded or inserted into slots therein, supports the magnetic field and couples that field to the winding, requiring significantly less magnetomotive force to maintain than air. It is a hitherto unsuspected discovery of the invention that a substantial magnetic field in a widened air gap, despite the relatively large magnetomotive force to maintain it, can be achieved provided a judicious selection of permanent magnets and efficacious utilization of that field by placing the stator winding into the gap is made.
The principal reliance of the prior art has been on the use of any permanent magnet material in conjunction with an iron laminated stator core. The magnetic flux density saturation of iron is greater than any known remanent magnetic flux density found in permanent magnet material, Alnicos having the highest known remanent field. The intent of the classical toothed iron core stator is to adjust the effective axially concentric cylindrical tooth cross-sectional areas such that a maximum magnetic flux is achieved providing for the total tooth cross-sectional area to be some fraction of the total axially concentric cylindrical area. This mechanism applied radially provides for the slots in which the winding is inserted. Consequently, this construction requires that the stator material saturation flux density be greater than the flux density operation point of the permanent magnet. Since the magnetic flux density saturation of magnetically soft ferrite materials is sometimes equal to but generally less than the remanent magnetic flux density of permanent magnets or generally favored operating points, the toothed stator core employing a magnetically soft ferrite is prohibited; and, for this reason, the two classes of materials is thought to be mutually incompatible.